How to retry after exception?

Question

I have a loop starting with for i in range(0, 100). Normally it runs correctly, but sometimes it fails due to network conditions. Currently I have it set so that on failure, it will continue in the except clause (continue on to the next number for i).

Is it possible for me to reassign the same number to i and run through the failed iteration of the loop again?

Answer 1

Do a while True inside your for loop, put your try code inside, and break from that while loop only when your code succeeds.

for i in range(0,100):
    while True:
        try:
            # do stuff
        except SomeSpecificException:
            continue
        break

Answer 2

I prefer to limit the number of retries, so that if there's a problem with that specific item you will eventually continue onto the next one, thus:

for i in range(100):
  for attempt in range(10):
    try:
      # do thing
    except:
      # perhaps reconnect, etc.
    else:
      break
  else:
    # we failed all the attempts - deal with the consequences.

Answer 3

The retrying package is a nice way to retry a block of code on failure.

For example:

@retry(wait_random_min=1000, wait_random_max=2000)
def wait_random_1_to_2_s():
    print("Randomly wait 1 to 2 seconds between retries")

Answer 4

Here is a solution similar to others, but it will raise the exception if it doesn't succeed in the prescribed number or retries.

tries = 3
for i in range(tries):
    try:
        do_the_thing()
    except KeyError as e:
        if i < tries - 1: # i is zero indexed
            continue
        else:
            raise
    break

Answer 5

The more "functional" approach without using those ugly while loops:

def tryAgain(retries=0):
    if retries > 10: return
    try:
        # Do stuff
    except:
        retries+=1
        tryAgain(retries)

tryAgain()

Answer 6

The clearest way would be to explicitly set i. For example:

i = 0
while i < 100:
    i += 1
    try:
        # do stuff

    except MyException:
        continue

Answer 7

Using recursion

for i in range(100):
    def do():
        try:
            ## Network related scripts
        except SpecificException as ex:
            do()
    do() ## invoke do() whenever required inside this loop

Answer 8

A generic solution with a timeout:

import time

def onerror_retry(exception, callback, timeout=2, timedelta=.1):
    end_time = time.time() + timeout
    while True:
        try:
            yield callback()
            break
        except exception:
            if time.time() > end_time:
                raise
            elif timedelta > 0:
                time.sleep(timedelta)

Usage:

for retry in onerror_retry(SomeSpecificException, do_stuff):
    retry()

Answer 9

There is something similar in the Python Decorator Library.

Please bear in mind that it does not test for exceptions, but the return value. It retries until the decorated function returns True.

A slightly modified version should do the trick.

Answer 10

for _ in range(5):
    try:
        # replace this with something that may fail
        raise ValueError("foo")

    # replace Exception with a more specific exception
    except Exception as e:
        err = e
        continue

    # no exception, continue remainder of code
    else:
        break

# did not break the for loop, therefore all attempts
# raised an exception
else:
    raise err

My version is similar to several of the above, but doesn't use a separate while loop, and re-raises the latest exception if all retries fail. Could explicitly set err = None at the top, but not strictly necessary as it should only execute the final else block if there was an error and therefore err is set.

Answer 11

Using while and a counter:

count = 1
while count <= 3:  # try 3 times
    try:
        # do_the_logic()
        break
    except SomeSpecificException as e:
        # If trying 3rd time and still error?? 
        # Just throw the error- we don't have anything to hide :)
        if count == 3:
            raise
        count += 1

Answer 12

You can use Python retrying package. Retrying

It is written in Python to simplify the task of adding retry behavior to just about anything.

Answer 13

If you want a solution without nested loops and invoking break on success you could developer a quick wrap retriable for any iterable. Here's an example of a networking issue that I run into often - saved authentication expires. The use of it would read like this:

client = get_client()
smart_loop = retriable(list_of_values):

for value in smart_loop:
    try:
        client.do_something_with(value)
    except ClientAuthExpired:
        client = get_client()
        smart_loop.retry()
        continue
    except NetworkTimeout:
        smart_loop.retry()
        continue

Answer 14

I use following in my codes,

   for i in range(0, 10):
    try:
        #things I need to do
    except ValueError:
        print("Try #{} failed with ValueError: Sleeping for 2 secs before next try:".format(i))
        time.sleep(2)
        continue
    break

Answer 15

i recently worked with my python on a solution to this problem and i am happy to share it with stackoverflow visitors please give feedback if it is needed.

print("\nmonthly salary per day and year converter".title())
print('==' * 25)


def income_counter(day, salary, month):
    global result2, result, is_ready, result3
    result = salary / month
    result2 = result * day
    result3 = salary * 12
    is_ready = True
    return result, result2, result3, is_ready


i = 0
for i in range(5):
    try:
        month = int(input("\ntotal days of the current month: "))
        salary = int(input("total salary per month: "))
        day = int(input("Total Days to calculate> "))
        income_counter(day=day, salary=salary, month=month)
        if is_ready:
            print(f'Your Salary per one day is: {round(result)}')
            print(f'your income in {day} days will be: {round(result2)}')
            print(f'your total income in one year will be: {round(result3)}')
            break
        else:
            continue
    except ZeroDivisionError:
        is_ready = False
        i += 1
        print("a month does'nt have 0 days, please try again")
        print(f'total chances left: {5 - i}')
    except ValueError:
        is_ready = False
        i += 1
        print("Invalid value, please type a number")
        print(f'total chances left: {5 - i}')

Answer 16

Here's my idea on how to fix this:

j = 19
def calc(y):
    global j
    try:
        j = j + 8 - y
        x = int(y/j)   # this will eventually raise DIV/0 when j=0
        print("i = ", str(y), " j = ", str(j), " x = ", str(x))
    except:
        j = j + 1   # when the exception happens, increment "j" and retry
        calc(y)
for i in range(50):
    calc(i)

Answer 17

increment your loop variable only when the try clause succeeds